A nigrosine dye such as C. I. Solvent Black 5 and C. I. Solvent Black 7 is an azine dye which is usually produced by condensation of aniline hydrochlorides with nitrobenzenes in the presence of catalysts such as iron chlorides.
The nigrosine dye is a mixture which comprises a number of azine compounds and by-products. The major components of the crude azine dye are blue-black dyes or purple-black dyes having a framework of phenazine, phenazineazine, triphenazineoxazine and the like.
Thus, the conventional nigrosine dye comprises the wide variety of colored or colorless by-products other than the major components. These by-products are responsible for the degradation in color strength, heat-resistance and light-resistance of the nigrosine dye, which impart detrimental effects on performance qualities. (i.e. the initial color strength, the aging stability, the fastness properties, the safety properties and the like) when used for many applications (e.g. a colorant of synthetic resin, a printing ink, a writing ink, a recording solution and a charge control agent). However, it has been believed that these by-products are the indispensable components for the color shade and other properties of the nigrosine dye.